I've never had a RD come up short, but I guess it's possible. Are you sure you have the limit screws backed out all the way? Or maybe it's something funky about where the cable is anchored?
I'd start with making really sure that the RD actually is stroke limited. Back the limit screw all the way out, unhook cable, move by hand.
Removing a sprocket is a so-so suggestion. Particularly removing an intermediate sprocket may cause clunky shifting, as the chain now has to deal with a bigger step. Shortening a sprocket at either end is usually a safer bet, particularly if you're willing to add spacers.
Besides, assuming that the RD really is limited in stroke, unless you really want the smallest cog, why not just leave it as it is? So there will be one sprocket there that won't take the chain, so what? Whether you've got 5 or 6 sprockets there, you've still got 5 useable ones.
And I can't say that I've tried it but I wonder if the thread on the uni-glide body extends far enough to lock the sprockets in place if you were simply to remove one.